keep (one's) hair on

To remain patient and calm, especially when beginning to become angry or upset with impatience. Usually said as an imperative. Primarily heard in UK. Yeah, yeah, keep your hair on! I'll have the report finished by the end of the day! A: "That lady is pestering us about getting her car back today." B: "We need another day to get it fixed. Tell her to keep her hair on!"

keep your hair on!

used to urge someone not to panic or lose their temper. British informal